The present invention relates to a process of pretreating an object prior to painting, and more particularly to a process of pretreating an object made of polyolefin such as polyethylene (PE) or polypropylene (PP) before it is coated with paint.
Polyolefin is widely used to make various products since it is lightweight, highly resistant to chemicals, and excellent in mechanical strength. Examples of such polyolefin products are automobile parts such as interior and exterior decorative members, bodies, and bumpers. Objects made of polyolefin are highly advantageous since various complex shapes of integral structure can be produced efficiently by a suitable molding process such as injection molding.
Although polyolefin articles are excellent in many respects, they have found only partial use as components that are required to be painted, such as automobile bodies and interior and exterior decorative parts, because a paint film coated directly on a polyolefin member for better appearance is poor in peel strength.
One process which has been proposed to improve the adhesion of a paint film to a polyolefin product is disclosed in U.S. Pat. No. 4,465,715. According to the disclosed pretreatment process, the surface of a polyolefin product prior to being coated with paint is washed with a chlorine-based or aromatic-group organic solvent having a solubility parameter (SP value) equal or similar to that of the polyolefin. Such solvents include trichloroethane, dichloroethylene, benzene, toluene, and the like. Plasma treating is thereafter performed on the surface of the product.
By treating the surface of a polyolefin product with an organic solvent having an SP value equal to or similar to that of the polyolefin, oil and polymers of lower grade can be removed from the surface of the product, and the surface can be swollen or made rough to increase its effective surface area. The plasma treating effected on the product treated with the solvent greatly increases the activation of the surface based on the production of a hydrophilic group. As an example described in the patent, polypropylene having an SP value ranging from 9.2 to 9.4 is used as the polyolefin, and trichloroethane having an SP value of 9.6 is used as the organic solvent.
The pretreatment process performed under the foregoing conditions has, however, failed to achieve the desired adhesion strength of the paint film.
This result appears to have been obtained for the following reasons:
Treating the surface of a polyolefin product with an organic solvent, as referred to above, having an SP value similar to that of polyolefin results in swelling and roughening of the product surface, and subsequent plasma treating should improve the surface activating effect. However, the surface layer of the product is rendered brittle due to the swelling thereof. Although the coating film firmly attaches to the swollen surface, the peel strength of the coating film is lowered. In the event that a chlorine-based organic solvent is used, there are certain handling problems particularly in wastewater treatment and process operation control.